Tread for an aircraft tire

ABSTRACT

An aeroplane tire, and, in particular, the tread thereof. The tread (2), having an axial width L, comprises a middle part (3) having an axial width LC at least equal to 50% and at most equal to 80% of the axial width L of the tread and composed of a middle rubber composition, and two lateral parts (41, 42), positioned axially on either side of the middle part (3), each having an axial width (LS1, LS2) at least equal to 10% and at most equal to 25% of the axial width L of the tread and each composed of a lateral rubber composition. The middle rubber composition comprises at least 50 phr of a first diene elastomer, a reinforcing filler and a crosslinking system, which first diene elastomer is a terpolymer of ethylene, of an α-olefin and of a non-conjugated diene.

RELATED APPLICATIONS

This is a U.S. National Phase Application under 35 USC 371 ofInternational Application PCT/FR2016/053007 filed on Nov. 18, 2016.

This application claims the priority of French application no. 1561129filed Nov. 19, 2015, the entire content of which is hereby incorporatedby reference.

FIELD OF THE INVENTION

The subject of the present invention is an aeroplane tire and, inparticular, the tread of an aeroplane tire.

BACKGROUND OF THE INVENTION

An aeroplane tire is characterized by use at high pressure, load andspeed. By way of example, an aeroplane tire of dimension 46×17R20,intended to be fitted to a commercial aeroplane, may be used at apressure equal to 15.3 bar, a static load equal to 21 tonnes and amaximum speed equal to 360 km/h. Generally, an aeroplane tire is used ata pressure of greater than 9 bar and a degree of deflection at leastequal to 32%. The use pressure is defined, for example, by the Tire andRim Association (TRA) standard. The degree of deflection of a tire is,by definition, its radial deformation, or its variation in radialheight, when the tire changes from an unladen inflated state to astatically loaded inflated state, under pressure and load conditionsrecommended, for example, by the TRA standard. It is expressed in theform of a relative deflection, defined by the ratio of this variation inradial height of the tire to half the difference between the outsidediameter of the tire and the maximum diameter of the rim measured on therim flange. The outside diameter of the tire is measured under staticconditions in an unladen state inflated to the recommended pressure.

Since a tire has a geometry that exhibits symmetry of revolution aboutan axis of rotation, the geometry of the tire is generally described ina meridian plane containing the axis of rotation of the tire. For agiven meridian plane, the radial, axial and circumferential directionsdenote the directions perpendicular to the axis of rotation of the tire,parallel to the axis of rotation of the tire and perpendicular to themeridian plane, respectively. The expressions “radially”, “axially” and“circumferentially” mean “in the radial direction”, “in the axialdirection” and “in the circumferential direction”, respectively.

The tread is the part of the tire intended to come into contact with theground via a tread surface, and extending radially from a bottom surfaceto the tread surface, axially from a first tread edge to a second treadedge defining the axial width of the tread, and circumferentially overthe whole periphery of the tire. Conventionally, the axial width of thetread is defined as the width of the patch of contact between the treadand the ground, measured along the axial straight line passing throughthe centre of the contact patch, when the new tire is subject to theloading and pressure conditions recommended by the TRA standard. Thetread is generally composed of raised elements separated by voids. Inthe case of an aeroplane tire, the raised elements are usuallycircumferential ribs, continuous over the whole circumference of thetire, and separated by circumferential voids or grooves. The tread,which is the wearing part of the tire, comprises at least one rubbercomposition, usually based on natural rubber and on carbon black, thesetwo main elements affording the rubber composition the mechanicalproperties necessary for the wear conditions of an aeroplane tire. Inaddition to these main elements, such a rubber compositionconventionally comprises a vulcanization system and protective agents.

Radially inside the tread, a radial-type tire comprises a reinforcement,consisting of a crown reinforcement and a radial carcass reinforcementradially inside the crown reinforcement. The crown reinforcementcomprises at least one crown layer composed of reinforcing elements orreinforcers coated with an elastomeric mixture and parallel to oneanother. The radial carcass reinforcement comprises at least one carcasslayer composed of reinforcers coated with an elastomeric mixture,parallel to one another and oriented substantially radially, that is tosay forming, with the circumferential direction, an angle of between 85°and 95°. The reinforcers of the crown and carcass layers, for aeroplanetires, are usually textile reinforcers made of aliphatic polyamide suchas nylon, made of aromatic polyamide such as aramid, or made of hybridmaterial combining, for example, an aliphatic polyamide and an aromaticpolyamide.

In aeroplane tires, the presence of non-uniform wear to the tread, knownas irregular wear, has been observed, resulting from the stresses thatoccur during the various life phases of the tire: take-off, taxiing andlanding. Differential wear to the tread between a middle part and thetwo lateral parts of the tread, axially on the outside of the middlepart, has more particularly been demonstrated, with the wear to thismiddle part being greater. The differential wear to the middle part ofthe tread leads to a limiting of the service life of the tire, andtherefore to a limiting of its use and to the premature removal thereof,despite the fact that the tread generally only has a relatively smalldegree of wear to the lateral parts of the tread: this is economicallyunsound.

Those skilled in the art have demonstrated two types of wear, dependingon the life phase of the tire. On landing, the middle part of the tread,having an axial width at least equal to 50% and at most equal to 80% ofthe total axial width of the tread, and coming into contact with theground, is subject to wear referred to as “touch wear”, resulting fromsignificant thermal heating at the moment at which the tread surfaceenters into contact with the ground, due to the speed differentialbetween the speed of rotation of the tire and the speed of theaeroplane. In the taxiing phase, before take-off or after landing, thelateral parts of the tread, positioned axially on either side of themiddle part and each having an axial width at least equal to 10% and atmost equal to 25% of the total axial width of the tread, are subject towear referred to as “taxiing wear”, resulting from the braking forcesexerted on these lateral parts due to their speed of rotation, which ishigher than that of the middle part. Thus, the tread is mainly worn inits middle part on landing and in its lateral parts on taxiing.

In order to solve the problem of irregular wear specific to aeroplanetires, those skilled in the art sought, according to a first approach,to optimize the inflated meridian profile of the tread surface, thismeridian profile being the meridian cross section through the treadsurface of an unladen new tire inflated to its nominal pressure, withouttaking into account circumferential grooves. Optimizing this inflatedmeridian profile, i.e. the geometric form thereof, makes it possible tooptimize the geometric form of the contact surface of the tire with theground and, consequently, the distribution of the mechanical stresseswithin this contact surface and hence to act on the wear of the tread.For example, documents EP 1163120, EP 1381525, EP 1477333 and EP 2310213describe solutions aiming to optimize the inflated profile of the treadsurface by acting on the tensile stiffnesses of the crown and/or carcasslayers, or on the tensile stiffness differentials between the middlepart and the lateral parts of the crown layers, or else on an optimizedcrown layer profile with a concave middle part. All these solutions arebased on changes in the material and/or geometry of the crown layers.

Another approach to the wear of an aeroplane tire is optimizing therubber composition(s) composing the tread. Indeed, wear also depends onthe rubber composition(s) composing the tread and on their sensitivityto abrasion, characterized in particular by their cohesion, sincecohesion depends on the chemical composition.

SUMMARY OF THE INVENTION

One object of the present invention, in relation to an aeroplane tire ofthe prior art, is to increase the resistance to touch wear of the middlepart of the tread during landing phases, while retaining the same levelof resistance to taxiing wear of the lateral parts of the tread duringtaxiing phases, by acting on the rubber composition(s) of the variousparts of the tread.

This object has been achieved, according to one aspect of the invention,by an aeroplane tire comprising a tread having an axial width L, thetread comprising:

a middle part having an axial width L_(C) at least equal to 50% and atmost equal to 80% of the axial width L of the tread and composed of amiddle rubber composition,

and two lateral parts positioned axially on either side of the middlepart, each having an axial width at least equal to 10% and at most equalto 25% of the axial width L of the tread and each composed of a lateralrubber composition,

the middle rubber composition comprising at least 50 phr of a firstdiene elastomer, a reinforcing filler and a crosslinking system, whichfirst diene elastomer is a terpolymer of ethylene, of an α-olefin and ofa non-conjugated diene.

It should be noted that the lateral parts of the tread may havedifferent axial widths and/or have different lateral rubbercompositions, even though, preferentially, the axial widths of thelateral parts are identical and though their lateral rubber compositionsare also identical.

A content by weight or content of a first diene elastomer at least equalto 50 phr (50 parts per hundred parts of elastomer) means that thisfirst diene elastomer is the predominant elastomer in the middle rubbercomposition. This predominant diene elastomer content provides a largecontribution to the touch wear resistance of the middle part of thetread.

Preferentially, the middle rubber composition comprises at least 60 phrof the first diene elastomer.

The α-olefin used for the synthesis of the terpolymer (or of the firstdiene elastomer) of the middle rubber composition may be a mixture ofα-olefins. The α-olefin generally comprises from 3 to 16 carbon atoms.Suitable as α-olefin are, for example, propylene, 1-butene, 1-pentene,1-hexene, 1-octene and 1-dodecene.

Advantageously, the α-olefin is propylene, in which case the terpolymeris commonly referred to as an EPDM (ethylene propylene diene monomer)rubber.

The non-conjugated diene used for the synthesis of the terpolymer (or ofthe first diene elastomer) of the middle rubber composition generallycomprises 6 to 12 carbon atoms. Mention may be made, as example ofnon-conjugated diene, of dicyclopentadiene, 1,4-hexadiene,5-ethylidene-2-norbornene, 5-methylene-2-norbornene or1,5-cyclooctadiene.

Advantageously, the non-conjugated diene is 5-ethylidene-2-norbornene ordicyclopentadiene.

According to one embodiment of the invention, the first diene elastomerof the middle rubber composition has at least one, and preferably all,of the following characteristics:

-   -   the ethylene units represent between 20 and 90%, preferentially        between 30 and 70%, by weight of the first diene elastomer,    -   the α-olefin units represent between 10 and 80%, preferentially        from 15 to 70%, by weight of the first diene elastomer,    -   the non-conjugated diene units represent between 0.5 and 20% by        weight of the first diene elastomer.

It is understood that the first diene elastomer may be composed of amixture of terpolymers of ethylene, of α-olefin and of non-conjugateddiene which differ from one another in their macrostructure or theirmicrostructure, in particular in the respective contents by weight ofthe ethylene, α-olefin and non-conjugated diene units.

According to a particular embodiment of the invention, the first dieneelastomer is the only elastomer of the middle rubber composition.

According to another particular embodiment of the invention, the middlerubber composition comprises a second elastomer, preferably a dieneelastomer, that is to say an elastomer comprising diene monomer units.When the middle rubber composition comprises a second elastomer, itpreferably comprises more than 50 phr, more preferentially more than 60phr, of the first diene elastomer.

The second elastomer of the middle rubber composition may be an“essentially unsaturated” or “essentially saturated” diene elastomer.“Essentially unsaturated” is understood to mean generally a dieneelastomer resulting at least in part from conjugated diene monomershaving a content of subunits or units of diene origin (conjugateddienes) which is greater than 15% (mol %); thus, diene elastomers suchas butyl rubbers or copolymers of dienes and of α-olefins of EPDM typedo not fall under the preceding definition and may especially bedescribed as “essentially saturated” diene elastomers (low or very lowcontent, always less than 15%, of subunits of diene origin). In thecategory of “essentially unsaturated” diene elastomers, “highlyunsaturated” diene elastomer is understood in particular to mean a dieneelastomer having a content of subunits of diene origin (conjugateddienes) which is greater than 50%.

Preferably, the second elastomer is a highly unsaturated diene elastomerselected from the group consisting of polybutadienes, polyisoprenes,butadiene copolymers, isoprene copolymers and mixtures of theseelastomers.

The polyisoprenes can be synthetic polyisoprenes (IR) or natural rubber(NR). It is understood that the second diene elastomer may be composedof a mixture of diene elastomers which differ from one another in theirmicrostructure, in their macrostructure, in the presence of a functionor in the nature or the position of the latter on the elastomer chain.

The reinforcing filler, known for its abilities to reinforce a rubbercomposition which can be used for the manufacture of tires, can be acarbon black, a reinforcing inorganic filler, such as silica, with whichis combined, in a known way, a coupling agent, or else a mixture ofthese two types of filler. Such a reinforcing filler typically consistsof nanoparticles, the (weight-)average size of which is less than amicrometre, generally less than 500 nm, most commonly between 20 and 200nm, in particular and more preferentially between 20 and 150 nm.

Advantageously, the reinforcing filler of the middle rubber compositioncomprises a carbon black.

The carbon black has a BET specific surface area preferably of at least90 m²/g, more preferentially of at least 100 m²/g. The blacksconventionally used in tires or their treads (“tire-grade” blacks) aresuitable for this purpose. Mention will more particularly be made, amongthe latter, of the reinforcing carbon blacks of the 100, 200 or 300series (ASTM grade), such as, for example, the N115, N134, N234 or N375blacks. The carbon blacks can be used in the isolated state, asavailable commercially, or in any other form, for example as support forsome of the rubber additives used. The BET specific surface area of thecarbon blacks is measured according to Standard D6556-10 [multipoint (atleast 5 points) method—gas: nitrogen—relative pressure p/p0 range: 0.1to 0.3].

According to a particular embodiment of the invention, the reinforcingfiller of the middle rubber composition comprises 100% by weight of acarbon black.

According to another embodiment of the invention, the reinforcing fillerof the middle rubber composition comprises an inorganic filler,preferably a silica.

The term “reinforcing inorganic filler” is intended to mean anyinorganic or mineral filler, regardless of its colour and its origin(natural or synthetic), also referred to as “white” filler, “clear”filler or even “non-black” filler, in contrast to carbon black, capableof reinforcing, by itself alone, without means other than anintermediate coupling agent, a rubber composition intended for themanufacture of tires, in other words capable of replacing, in itsreinforcing role, a conventional tire-grade carbon black; such a filleris generally characterized, in a known way, by the presence of hydroxyl(—OH) groups at its surface.

Mineral fillers of the siliceous type, preferentially silica (SiO₂), areespecially suitable as reinforcing inorganic fillers. The silica usedmay be any reinforcing silica known to those skilled in the art,especially any precipitated or fumed silica exhibiting a BET surfacearea and a CTAB specific surface area both of less than 450 m²/g,preferably from 30 to 400 m²/g and especially between 60 and 300 m²/g.

The physical state in which the reinforcing inorganic filler is providedis unimportant, whether it is in the form of a powder, microbeads,granules or else beads. Of course, the term “reinforcing inorganicfiller” is also intended to mean mixtures of various reinforcinginorganic fillers, in particular of highly dispersible silicas asdescribed above.

In the present account, as regards the silica, the BET specific surfacearea is determined in a known way by gas adsorption using theBrunauer-Emmett-Teller method described in The Journal of the AmericanChemical Society, Vol. 60, page 309, February 1938, more specificallyaccording to French Standard NF ISO 9277 of December 1996 (multipoint (5point) volumetric method—gas: nitrogen—degassing: 1 hour at 160°C.—relative pressure p/po range: 0.05 to 0.17). The CTAB specificsurface area is the external surface determined according to FrenchStandard NF T 45-007 of November 1987 (method B).

In order to couple the reinforcing inorganic filler to the dieneelastomer, use is made, in a well-known way, of an at least bifunctionalcoupling agent (or bonding agent) intended to provide a satisfactoryconnection, of chemical and/or physical nature, between the inorganicfiller (surface of its particles) and the diene elastomer. Use is madein particular of at least bifunctional organosilanes orpolyorganosiloxanes.

Advantageously, the content of reinforcing filler of the middle rubbercomposition is at least equal to 20 phr and at most equal to 70 phr,preferably at least equal to 25 phr and at most equal to 50 phr.

In particular, a content of reinforcing filler of the middle rubbercomposition preferentially at least equal to 25 phr and at most equal to50 phr makes it possible to improve the resistance to touch wear duringlandings, without degrading the resistance to taxiing wear duringtaxiing phases.

The crosslinking system can be based either on sulfur or on sulfurdonors and/or on peroxide and/or on bismaleimides. The crosslinkingsystem is preferentially a vulcanization system, i.e. a system based onsulfur (or on a sulfur-donating agent) and on a primary vulcanizationaccelerator. To this base vulcanization system, various known secondaryvulcanization accelerators or vulcanization activators are added, suchas zinc oxide, stearic acid or equivalent compounds, or guanidinederivatives (in particular diphenylguanidine), or else knownvulcanization retarders, which are incorporated during the firstnon-productive phase and/or during the productive phase, as describedsubsequently.

The sulfur is used at a preferential content of between 0.5 and 12 phr,in particular between 1 and 10 phr. The primary vulcanizationaccelerator is used at a preferential content of between 0.5 and 10 phr,more preferentially of between 0.5 and 5.0 phr.

The middle rubber composition may also comprise all or a portion of theusual additives customarily used in elastomer compositions intended toconstitute treads, such as, for example, plasticizers, pigments,protective agents, such as antiozone waxes, chemical antiozonants orantioxidants, or antifatigue agents.

Advantageously, the middle rubber composition comprises 0 to 20 phr of aliquid plasticizer.

A plasticizer is regarded as being liquid when, at 23° C., it has theability to ultimately assume the shape of its container, this definitionbeing given in contrast to plasticizing resins, which are by naturesolid at ambient temperature. Mention may be made, as liquidplasticizer, of vegetable oils, mineral oils, ether, ester, phosphate orsulfonate plasticizers, and their mixtures.

Preferentially, the content of liquid plasticizer of the middle rubbercomposition is equal to 0.

At least one lateral rubber composition is advantageously different fromthe middle rubber composition.

According to a first embodiment, at least one lateral rubber compositionadvantageously comprises a diene elastomer, a reinforcing filler and acrosslinking system, which diene elastomer is a highly unsaturated dieneelastomer, selected from the group consisting of polybutadienes,polyisoprenes, butadiene copolymers, isoprene copolymers and themixtures of these elastomers.

This first embodiment of the lateral rubber composition is identical tothe rubber composition of a tread according to the prior art andtherefore guarantees a level of resistance to wear, in the taxiingphase, of the lateral parts of the tread that is identical to that ofthe prior art, used as reference.

According to a second embodiment, at least one lateral rubbercomposition advantageously comprises at most 50 phr of the first dieneelastomer according to any one of the embodiments of the first dieneelastomer described above. The diene elastomer of a lateral rubbercomposition may or may not be identical to the first diene elastomer ofthe middle rubber composition.

This second embodiment of the lateral rubber composition guarantees alevel of resistance to wear, in the taxiing phase, of the lateral partsof the tread that is close to that of the prior art, used as reference.

Usually, the two lateral parts, positioned axially on either side of themiddle part, have identical axial widths. Advantageously, the twolateral parts are composed of identical lateral rubber compositions.According to a preferred embodiment, the two lateral parts, positionedaxially on either side of the middle part, have identical axial widthsand are composed of identical lateral rubber compositions.

Since the tire comprises a crown reinforcement radially inside thetread, the tire advantageously comprises an interlayer composed of arubber composition, in contact by a radially outer face with at leastthe middle portion of the tread and by a radially inner face with thecrown reinforcement. Contact of the radially outer face of theinterlayer with at least the middle portion of the tread means that theaxial width of this contact is at least equal to the axial width L_(C)of the middle portion of the tread. Contact of the radially inner faceof the interlayer with the crown reinforcement is contact with theprotective reinforcement, which is the radially outermost part of thecrown reinforcement, intended to protect the working reinforcement,which is the radially innermost part of the crown reinforcement. Thisinterlayer, also referred to as connecting layer, guarantees betterconnection between the tread comprising a rubber composition accordingto the invention and the crown reinforcement.

According to a first embodiment, the interlayer is composed of a rubbercomposition comprising natural rubber. The rubber composition alsocomprises a reinforcing filler and a crosslinking system.

According to a second embodiment, the interlayer is composed of a rubbercomposition comprising an elastomer matrix, which elastomer matrixcontains a terpolymeric elastomer of ethylene, of an α-olefin and of anon-conjugated diene and contains at least 10% by weight of diene units.“Elastomer matrix” is used to refer to all the elastomers contained inthe rubber composition. The rubber composition also comprises areinforcing filler and a crosslinking system. It is understood that theelastomer may be a mixture of terpolymers of ethylene, of α-olefin andof non-conjugated diene which differ from one another in theirmacrostructure or their microstructure, in particular by the respectivecontents by weight of the ethylene, α-olefin and non-conjugated dieneunits. A diene unit is a monomer unit originating from the insertion ofa monomer subunit resulting from the polymerization of a conjugateddiene monomer or of a non-conjugated diene monomer, the diene unitcomprising a carbon-carbon double bond. The rubber composition of theinterlayer, according to this second embodiment, may be used in itsgeneric form, as described above, or in the form of any one of itsembodiments, described in document FR 14/61754.

According to a third embodiment, the interlayer is composed of a rubbercomposition comprising an elastomer comprising ethylene units and dieneunits comprising a carbon-carbon double bond, which units aredistributed randomly within the elastomer. The rubber composition alsocomprises a reinforcing filler and a crosslinking system. The rubbercomposition of the interlayer, according to this third embodiment, maybe used in its generic form, as described above, or in the form of anyone of its embodiments, described in document FR 14/61755.

According to a fourth embodiment, the interlayer is composed of anelastomeric laminate comprising, radially from the outside to theinside, n layers Ci, n being an integer greater than or equal to 2 and ibeing an integer ranging from 1 to n, each composed of a diene rubbercomposition, the layer C1 comprising a diene elastomer E comprisingethylene units and diene units, the diene units representing more than10% by weight of the monomer units of the diene elastomer E, the layerCn comprising from 50 to less than 100 phr of a diene elastomer N havinga content by weight of diene units of greater than 50%, the contentexpressed in phr of diene elastomer N being higher in the layer Cn thanin the layer C1, the content expressed in phr of diene elastomer E beinghigher in the layer C1 than in the layer Cn, the layers Ci, for thevalues of i ranging from 2 to n−1, where n is greater than 2, comprisinga diene elastomer I selected from the group consisting of dienehomopolymers and copolymers having more than 10% by weight of dieneunits. The rubber compositions also comprise a reinforcing filler and acrosslinking system. The rubber compositions of the interlayer,according to this fourth embodiment, may be used in their generic forms,as described above, or in the form of any one of their respectiveembodiments, described in document FR 14/62227.

Regarding the composition of the elastomers, the microstructure isgenerally determined by ¹H NMR analysis, supplemented by ¹³C NMRanalysis when the resolution of the ¹H NMR spectra does not enable theattribution and quantification of all the species. The measurements arecarried out using a Bruker 500 MHz NMR spectrometer at frequencies of500.43 MHz for observing protons and 125.83 MHz for observing carbons.For the measurements of mixtures or elastomers which are insoluble butwhich have the ability to swell in a solvent, an HRMAS z-grad 4 mm probeis used, making it possible to observe protons and carbons inproton-decoupled mode. The spectra are acquired at spin speeds of 4000Hz to 5000 Hz. For the measurements of soluble elastomers, a liquid NMRprobe is used, making it possible to observe protons and carbons inproton-decoupled mode. The insoluble samples are prepared in rotorsfilled with the analyte and a deuterated solvent enabling swelling, ingeneral deuterated chloroform (CDCl₃). The solvent used must always bedeuterated and its chemical nature may be adapted by those skilled inthe art. The amounts of material used are adjusted so as to obtainspectra with sufficient sensitivity and resolution. The soluble samplesare dissolved in a deuterated solvent (approximately 25 mg of elastomerin 1 ml), in general deuterated chloroform (CDCl₃). The solvent orsolvent blend used must always be deuterated and its chemical nature maybe adapted by those skilled in the art. The sequences used for protonNMR and carbon NMR, respectively, are identical for a soluble sample andfor a swelled sample. For the proton NMR, a simple 30° pulse sequence isused. The spectral window is adjusted to observe all the resonance linesbelonging to the molecules analysed. The accumulation number is adjustedin order to obtain a signal to noise ratio that is sufficient for thequantification of each subunit. The recycle period between each pulse isadapted to obtain a quantitative measurement. For the carbon NMR, asimple 30° pulse sequence is used with proton decoupling only duringacquisition to avoid the “nuclear Overhauser” effects (NOE) and toremain quantitative. The spectral window is adjusted to observe all theresonance lines belonging to the molecules analysed. The accumulationnumber is adjusted in order to obtain a signal to noise ratio that issufficient for the quantification of each subunit. The recycle periodbetween each pulse is adapted to obtain a quantitative measurement. TheNMR measurements are carried out at 25° C.

BRIEF DESCRIPTION OF THE DRAWINGS

The characteristics of the invention will be better understood by meansof FIGS. 1 and 2, and by means of the results of measurements and testscarried out on rubber compositions as used in a tire according to theinvention and the results of tests carried out on tires according to theinvention.

FIG. 1 shows a view in cross-section in a meridian plane of the crown ofan airplane tire according to an embodiment of the invention.

FIG. 2 shows a view in cross-section in a meridian plane of the crown ofan airplane tire according to another embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1, not shown to scale in order to facilitate the understandingthereof, presents a view in cross section in a meridian plane of thecrown of an aeroplane tire according to the invention, comprising,radially from the outside to the inside, a tread 2, a crownreinforcement 5 and a carcass reinforcement 6. The tread 2, having anaxial width L, comprises a middle part 3 having an axial width L_(C) atleast equal to 50% and at most equal to 80% of the axial width L of thetread and composed of a middle rubber composition, and two lateral parts(41, 42), positioned axially on either side of the middle part 3, eachhaving an axial width (L_(S1), L_(S2)) at least equal to 10% and at mostequal to 25% of the axial width L of the tread and each composed of alateral rubber composition.

FIG. 2 presents a view in cross section in a meridian plane of the crownof an aeroplane tire according to a particular embodiment of theinvention, wherein the tire 1 also comprises an interlayer 7 composed ofa rubber composition, in contact by a radially outer face with the tread2 and by a radially inner face with the crown reinforcement 5.

The invention has more particularly been studied in the case of anaeroplane tire of dimension 46×17R20, intended to be fitted to the mainlanding gear of a commercial airliner. For such a tire, the inflationpressure is 15.3 bar, the static load is 21 tonnes and the maximum speedis 360 km/h.

Laboratory tests and measurements were carried out on different rubbercompositions comprising a terpolymeric diene elastomer of ethylene, ofan α-olefin and of a non-conjugated diene, in comparison with rubbercompositions based on natural rubber which are generally used inaeroplane tire treads of the prior art.

The rubber compositions according to the invention and of the prior artwere prepared according to the process described below. The dieneelastomers, the reinforcing fillers and also the various otheringredients, with the exception of the vulcanization system, aresuccessively introduced into an internal mixer (final degree of filling:approximately 70% by volume), the initial vessel temperature of which isapproximately 80° C. Thermomechanical working (non-productive phase) isthen carried out in one step, which lasts in total approximately 3 to 4min, until a maximum “dropping” temperature of 165° C. is reached. Themixture thus obtained is recovered and cooled and then sulfur and anaccelerator of sulfamide type are incorporated on a mixer (homofinisher)at 70° C., everything being mixed (productive phase) for an appropriatetime (for example approximately ten minutes). The compositions thusobtained are subsequently calendered, either in the form of slabs(thickness of 2 to 3 mm) or of thin sheets of rubber, for themeasurement of their physical or mechanical properties, or extruded inthe form of an aeroplane tire tread.

The wear resistance of the rubber compositions defined above wasevaluated on samples, in particular by a high-speed abrasion test,representative of the landing conditions of an aeroplane tire, combinedwith a measurement of loss in weight and by a measurement of breakingstrength.

Regarding the loss in weight, a sample of rubber composition issubjected to an abrasion test on a high-speed abrasion tester. Thehigh-speed abrasion test is carried out according to the principledescribed in the paper by S. K. Clark, “Touchdown dynamics”, PrecisionMeasurement Company, Ann Arbor, Mich., NASA, Langley Research Center,Computational Modeling of Tires, pages 9-19, published in August 1995.The tread material rubs over a surface, such as a Norton VulcanA30S-BF42 disc. The linear speed during contact is 70 m/s with a meancontact pressure of 15 to 20 bar. The device is designed to rub untilexhausting of the energy from 10 to 20 MJ/m² of contact surface. Theloss in weight performance is evaluated on the basis of the loss inweight according to the following formula: Loss in weightperformance=loss in weight control/loss in weight sample. The resultsare expressed in base 100. A loss in weight performance for the sampleof greater than 100 is regarded as better than the control.

In tables 1 and 2 presented below, the rubber compositions T1 and T2 aretwo rubber compositions of the prior art, used as reference. The rubbercomposition T1 corresponds to a composition based on natural rubber,commonly used by those skilled in the art to manufacture an aeroplanetire tread. The rubber composition T2 also contains natural rubber, butwith a content of filler and a vulcanization system which differ fromthose of the rubber composition T1.

The rubber compositions C1 to C5, C15, C18 to C24 contain an EPDM dieneelastomer, a reinforcing filler comprising a carbon black and/or asilica at different contents, and a crosslinking system. They differ bythe content of EPDM diene elastomer and by the nature and the content ofreinforcing filler (carbon black or silica).

A first test, the results of which are presented in table 1 below, hasthe aim of showing the influence of the content of EPDM diene elastomerin the rubber composition on elongation at break and loss in weight.

TABLE 1 T2 C1 C2 C3 C4 C5 NR (1) 100 — 10 20 40 60 EPDM 1 (2) — 100 9080 60 40 Carbon black (3) 30 30 30 30 30 30 Antioxidant (4) 1.5 1.5 1.51.5 1.5 1.5 Stearic acid (5) 2.5 2.5 2.5 2.5 2.5 2.5 Zinc oxide (6) 3 33 3 3 3 Accelerator (7) 2 2 2 2 2 2 Sulfur 0.8 0.8 0.8 0.8 0.8 0.8Elongation at break 528 634 664 658 560 465 at 23° C. (%) Loss in weight100 173 146 132 123 119 performance (%) (1) Natural rubber (2) EPDM,Nordel IP 4570 from Dow (3) Carbon black of N234 grade according toStandard ASTM D-1765 (4)N-(1,3-Dimethylbutyl)-N-phenyl-para-phenylenediamine: Santoflex 6-PPDfrom Flexsys (5) Stearin, Pristerene 4931 from Uniqema (6) Zinc oxide ofindustrial grade from Umicore (7)N-Cyclohexyl-2-benzothiazolesulfenamide, Santocure CBS from Flexsys

The result of this first test shows that the loss in weight performanceof the rubber compositions C1 to C5 is always improved relative to thatof the reference rubber composition T2. In other words, the losses inweight for the rubber compositions C1 to C5 are always less than that ofthe rubber composition T2, with the difference in loss in weightperformance being able to reach +73% in the case of the rubbercomposition C1 comprising 100% EPDM. Regarding the elongation at break,it is greater than the reference for the rubber compositions C1 to C4,but becomes less than the reference for the rubber composition C5, inwhich the EPDM content is less than 50 phr. It is observed that the useof more than 50 phr of EPDM in the rubber composition results in abetter compromise in performance between the loss in weight and theelongation at break. Thus, the invention has the advantage ofguaranteeing a better loss in weight performance, representative of abetter wear resistance during the phase of landing the aeroplane.

A second test, the results of which are presented in table 2 below, hasthe aim of showing the influence of the nature and the content ofreinforcing filler in the rubber composition on the loss in weight.

TABLE 2 T1 C1 C15 C18 C19 C20 C21 C22 C23 C24 NR (1) 100 — — — — — — — —— EPDM (2) — 100 100 100 100 100 100 100 100 100 Carbon black 1 (3) 47.530 47.5 70 — — — — — — Carbon black 2 (4) — — — — 30 47.5 — — — — Carbonblack 3 (5) — — — — — — 30 47.5 — — Silica (6) — — — — — — — — 30 47.5Silane (7) — — — — — — — — 2.4 3.8 Antioxidant (8) 1.5 1.5 1.5 1.5 1.51.5 1.5 1.5 1.5 1.5 Stearic acid (9) 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.52.5 Zinc oxide (10) 3 3 3 3 3 3 3 3 3 3 Accelerator (11) 0.8 2 2 2 2 2 22 0.8 0.8 Sulfur 1.5 0.8 0.8 0.8 0.8 0.8 0.8 0.8 1.5 1.5 Loss in weight100 195 149 112 184 151 182 153 157 126 performance (%) (1) Naturalrubber (2) EPDM, Nordel IP 4570 from Dow (3) Carbon black of N234 gradeaccording to Standard ASTM D-1765 (4) Carbon black of N115 gradeaccording to Standard ASTM D-1765 (5) Carbon black of N550 gradeaccording to Standard ASTM D-1765 (6) Silica of 160 MP grade (7) Liquidsilane, Si69 from Degussa (8)N-(1,3-Dimethylbutyl)-N-phenyl-para-phenylenediamine, Santoflex 6-PPDfrom Flexsys (9) Stearin, Pristerene 4931 from Uniqema (10) Zinc oxideof industrial grade from Umicore (11)N-Cyclohexyl-2-benzothiazolesulfenamide, Santocure CBS from Flexsys

The result of this second test shows that the loss in weight performancefor the rubber compositions according to the invention, C1, C15, C18 toC24 is always improved relative to the reference rubber composition Ti.It is also observed that carbon black, especially at a content of lessthan 70 phr, leads to a better result than silica.

In summary, the rubber compositions based on at least one terpolymer ofethylene, of an α-olefin and of a non-conjugated diene, a reinforcingfiller and a crosslinking system, which constitute the middle part ofthe tread of an aeroplane tire, afford the tire greatly improvedperformance in terms of resistance to touch wear during landing.

Comparative tests were also carried out between a tire according to theinvention, of dimension 46×17R20, and a reference tire of the samedimension. The tested tire according to the invention comprises a treadcomposed of the rubber composition C1, defined in table 2 and comprising100 phr of EPDM and 30 phr of carbon black N234. The tested referencetire comprises a tread composed of the rubber composition T1 describedabove.

A first test consisted in simulating landings, on a suitable testingmeans, until the tread was entirely worn, and in measuring the loss inweight corresponding to the wearing of the tread upon removal of theentirely worn tire. The reference tire was thus removed after 375landings, whereas the tire according to the invention was removed after675 landings. In other words, the tire according to the inventionperformed 1.8 times the number of landings of the reference tire beforeit was totally worn and removed.

A second test consisted in simulating, on a suitable testing means,running representative of the taxiing phase over a determined distanceand in measuring the loss in weight corresponding to the wear of thetread at this distance. After approximately 700 km of running, therespective losses in weight of the reference tire and the tire accordingto the invention were observed to be at substantially the same level.This lack of degradation of the resistance to taxiing wear isastonishing, since, for those skilled in the art, a lower content ofreinforcing filler—in the example tested, 30 phr of N234 carbon blackfor the tire according to the invention, instead of 47.5 phr of N234carbon black for the reference tire—should have degraded the wearresistance.

On the basis of the results of these comparative wear tests, theinventors consider that a tire according to the invention, compared tothe reference tire, enables an overall gain in wear life over the wholecycle of use of the tire, comprising the phases of landing, taxiing andbraking.

This gain in wear life of the tire according to the invention, obtainedby virtue of a more regular wearing of the tread, also presents anadvantage in terms of retreading the tire, that is to say replacing theworn tread of the tire at the end of life.

For a tire of the prior art at the end of life, for which the tread hasa wear differential between the middle part and the lateral parts, theretreading operation commonly requires, aside from the removal of theworn tread, the removal of the radially outermost crown layer, generallycomposed of metal reinforcers and referred to as protective layer, saidprotective layer often being damaged at the end of life of the tire dueto its proximity to the tread.

For a tire according to the invention, due to a more regular wearingover the axial width of the tread, removal of the protective layer is nolonger necessary due to its integrity at the end of life of the tire,which gives rise to an economic gain in the retreading operation.

The scope of protection of the invention is not limited to the examplesgiven hereinabove. The invention is embodied in each novelcharacteristic and each combination of characteristics, which includesevery combination of any features which are stated in the claims, evenif this feature or combination of features is not explicitly stated inthe examples.

The invention claimed is:
 1. An airplane tire comprising a tread havingan axial width L, the tread comprising: a middle part having an axialwidth L_(C) at least equal to 50% and at most equal to 80% of the axialwidth L of the tread and composed of a middle rubber composition, andtwo lateral parts positioned axially on either side of the middle part,each having an axial width at least equal to 10% and at most equal to25% of the axial width L of the tread and each composed of a lateralrubber composition, wherein the middle rubber composition comprises afirst diene elastomer as the only elastomer, a reinforcing filler and acrosslinking system, which first diene elastomer is a terpolymer ofethylene, of an α-olefin and of a non-conjugated diene, and wherein theα-olefin is propylene, and wherein at least one lateral rubbercomposition comprises a diene elastomer, a reinforcing filler and acrosslinking system, which diene elastomer is a highly unsaturated dieneelastomer, selected from the group consisting of polybutadienes,polyisoprenes, butadiene copolymers, isoprene copolymers and themixtures of these elastomers.
 2. The tire according to claim 1, whereinthe non-conjugated diene is 5-ethylidene-2-norbornene ordicyclopentadiene.
 3. The tire according to claim 1, wherein the firstdiene elastomer has at least one, of the following characteristics: theethylene units represent between 20 and 90% by weight of the first dieneelastomer, the α-olefin units represent between 10 and 80% by weight ofthe first diene elastomer, the non-conjugated diene units representbetween 0.5 and 20% by weight of the first diene elastomer.
 4. The tireaccording to claim 1, wherein the reinforcing filler of the middlerubber composition comprises a carbon black.
 5. The tire according toclaim 4, wherein the reinforcing filler of the middle rubber compositioncomprises 100% by weight of a carbon black.
 6. The tire according toclaim 1, wherein the reinforcing filler of the middle rubber compositioncomprises an inorganic filler.
 7. The tire according to claim 1, whereinthe content of reinforcing filler of the middle rubber composition is atleast equal to 20 phr and at most equal to 70 phr.
 8. The tire accordingto claim 1, wherein the middle rubber composition comprises from 0 to 20phr of a liquid plasticizer.
 9. The tire according to claim 8, whereinthe content of liquid plasticizer of the middle rubber composition isequal to
 0. 10. The tire according to claim 1, wherein at least onelateral rubber composition is different from the middle rubbercomposition.
 11. The tire according to claim 1, wherein at least onelateral rubber composition comprises at most 50 phr of the first dieneelastomer.
 12. The tire according to claim 1, wherein the two lateralparts, positioned axially on either side of the middle part, haveidentical axial widths and are composed of identical lateral rubbercompositions.
 13. The tire according to claim 1, the tire comprising acrown reinforcement radially inside the tread, wherein the tirecomprises an interlayer composed of at least one rubber composition, incontact by a radially outer face with at least the middle part of thetread and by a radially inner face with the crown reinforcement.
 14. Thetire according to claim 13, wherein the interlayer is composed of arubber composition comprising natural rubber.
 15. The tire according toclaim 13, wherein the interlayer is composed of a rubber compositioncomprising an elastomer matrix, which elastomer matrix contains aterpolymeric elastomer of ethylene, of an α-olefin and of anon-conjugated diene and contains at least 10% by weight of diene units.16. The tire according to claim 13, wherein the interlayer is composedof a rubber composition comprising an elastomer comprising ethyleneunits and diene units comprising a carbon-carbon double bond, whichunits are distributed randomly within the elastomer.
 17. The tireaccording to claim 13, wherein the interlayer is composed of anelastomeric laminate comprising, radially from the outside to theinside, n layers Ci, n being an integer greater than or equal to 2 and ibeing an integer ranging from 1 to n, each composed of a diene rubbercomposition, the layer C1 comprising a diene elastomer E comprisingethylene units and diene units, the diene units representing more than10% by weight of the monomer units of the diene elastomer E, the layerCn comprising from 50 to less than 100 phr of a diene elastomer N havinga content by weight of diene units of greater than 50%, the contentexpressed in phr of diene elastomer N being higher in the layer Cn thanin the layer C1, the content expressed in phr of diene elastomer E beinghigher in the layer C1 than in the layer Cn, the layers Ci, for thevalues of i ranging from 2 to n−1, where n is greater than 2, comprisinga diene elastomer I selected from the group consisting of dienehomopolymers and copolymers having more than 10% by weight of dieneunits.